A convex formed portion is typically formed by performing a pushing process such as drawing using a surface treated metal plate such as a coated steel plate as a raw material. When the formed portion requires a particularly high degree of dimensional precision, ironing is implemented on the formed portion after the formed portion is formed. Ironing is a processing method of setting a clearance between a punch and a die to be narrower than a plate thickness of the formed portion prior to ironing, and then ironing a plate surface of the formed portion using the punch and the die so that the plate thickness of the formed portion matches the clearance between the punch and the die.
A configuration disclosed in Patent Document 1 and so on, shown below, for example, may be employed as a mold used during ironing. Specifically, the conventional mold includes a punch and a die. The punch is a columnar member having an outer peripheral surface that extends rectilinearly parallel to a pushing direction into a pushing hole, and is inserted into a formed portion. The die includes the pushing hole into which the formed portion is pushed together with the punch. The pushing hole has a shoulder portion disposed on an outer edge of an inlet of the pushing hole and constituted by a curved surface having a predetermined curvature radius, and an inner peripheral surface that extends rectilinearly from a radius end of the shoulder portion parallel to the pushing direction. When the formed portion is pushed into the pushing hole, the plate surface thereof is ironed by the shoulder portion so as to decrease gradually in thickness to a width of a clearance between the outer peripheral surface of the punch and the inner peripheral surface of the pushing hole.